Aluminum nitride can be prepared by a number of methods. Previous methods of preparation are briefly discussed below. All figures given in percent herein are weight percent.
One method utilizes carbothermic reduction of alumina under a nitrogen atmosphere according to the following reaction EQU Al.sub.2 O.sub.3 +3C+N.sub.2 .fwdarw.2AlN+3CO (1)
This process is said to produce 98 percent AlN continuously. The major impurities of the product are said to be oxygen and carbon. Cooper, C. F., George, C. M., and Hopkins, S. W. J., "Preparation and Oxidation of Aluminum Nitride", In Special Ceramics 1962, Ed. by P. Popper, Published by the British Ceramic Association (1963).
Russian Pat. No. 217,298 teaches the production of aluminum nitride from aluminum containing minerals with carbon in an N.sub.2 atmosphere in the presence of sulfur. Sulfur is used in the carbothermic reduction to assist in the removal of metallic impurities such as silicon and titanium. The amount of sulfur should not exceed that required to remove the metallic impurities as monosulfides to prevent the sulfur from reacting with the aluminum.
Shalom tried to use the same reaction to obtain aluminum oxynitride (AlON; 5AlN.9Al.sub.2 O.sub.3). The product obtained, however, was always on impure AlN when the reactants were mixed in stoichiometric amount (i.e., one Al.sub.2 O.sub.3 to three C). The major impurity was again carbon. Moshe Ish-Shalom, "Formation of Aluminum Oxynitride by Carbothermal Reduction of Aluminum Oxide in Nitrogen", J. Material Science Letters, 1, 147-149 (1982). Kuramoto and Taniguchi also used the same reaction but they were able to reduce the carbon impurity to 0.15 percent level by a careful low temperature oxidation process. The resulting powder, however, contained less than 240 ppm metallic impurities and about 1.0 percent oxygen. Nevertheless, the powder was hot pressed or sintered to greater than 99 percent of the density using CaO as sintering aid. N. Kuramoto, H. Taniguchi, "Transparent AlN Ceramics", Ibid. 3, 471-474 (1984).
In U.S. Pat. No. 3,307,908 to Mandorf, aluminum metal in a finely divided carrier material such as AlN, AlF, or a mixture of the two Mandorf teaches against the presence of H.sub.2 S since it interferes with the nitriding reaction.
To prepare ultrafine AlN powder and to avoid the carbon impurity Hock and Nair used Al(OH).sub.3 and amorphous Al.sub.2 O.sub.3 powders prepared from the hydrolysis of aluminum alkoxide to react with NH.sub.3 EQU Al.sub.2 O.sub.3 +2NH.sub.3 .fwdarw.2AlN+3H.sub.2 O (2)
To avoid grain growth, the reaction was carried out at low temperatures (between 800 to 1350 C.). Although the reaction produced AlN, the amount present in the product was generally less than 80 percent. The main impurities were various polymorphs of Al.sub.2 O.sub.3. Hoch, M. and Manikantar Nair, K., "Preparation and Characterization of Ultrafine Powders of Refractory Nitrides: I, AlN and Si.sub.3 N.sub.4 ", Cer. Soc. Bull 58, [2], 187-190 (1979).
U.I. Myakinenkov et al teaches the use of volatile compounds of aluminum with nitrogen compounds to form films of AlN. The particular system described uses an organic compound of aluminum (triethyl aluminum, boiling point 194 C.) with hydrazine in a carrier gas. Reaction temperatures are 750 to 1100 C. Inorganic Materials, Vol. 10, No. 10, p. 1635-1636, October 1974 (Publ. March 1975).
Direct reaction of aluminum with nitrogen according to the reaction EQU 2Al+N.sub.2 .fwdarw.2AlN (3)
is an exothermic reaction. This reaction is particularly suitable for large scale AlN production. The product of reaction (3) is a highly sintered mass. As a result, the grinding of the product is required. This grinding process itself can introduce impurities. Another common impurity of this reaction is unreacted aluminum metal. Cooper et al., used about 1 weight percent LiF as a catalyst to improve the kinetics of reaction (3). LiF was most effective compared to NaF, KHF.sub.2 and AlF.sub.3. Although they obtained relatively pure AlN powder (99.7 AlN that contained 0.009 percent free Al) the powders were not sinterable. Cooper, C. F., George, C. M., and Winter, L., Aluminum Nitride Crucibles: Raw Materials Preparation, Characterization and Fabrication, in Special Ceramics, Vol. 4., Ed. by P. Popper, Pub. by the British Ceramic Research Association, 1-3 (1968).
In earlier work Cooper et al (1963 above) used reaction (3) to obtain 99.8 to 99.9 percent pure AlN by levitating the reacting pear-shaped high purity aluminum ball in an electromagnetic field. A variation of the method involves vaporizing aluminum by striking a d.c. arc between two aluminum electrodes in a nitrogen atmosphere. About 80 percent of the AlN product forms as hard lumps on the electrode surfaces, the remaining 20 percent deposited as a fine, reactive powder in the reaction chamber. The analysis of the lumps showed that product contained 92 to 94 percent AlN. The fine powder contained about 25 percent AlN. The remaining portions considered to be free aluminum and Al.sub.2 O.sub.3 impurities which are presumed to form as a result of O.sub.2 leakage into the chamber. Heat treating these powders in vacuum at high temperature reduced their oxygen impurity and their reactivity to moisture. Long, G. and Foster, L. M., "Aluminum Nitride, Refractory for Aluminum to 2000 C.", J. Amer. Soc. 42, 53-59, ( 1959).
Sato and Iwata used the same technique of Long and Foster but they varied the nitrogen pressure. The maximum yield obtained (about 0.6 grams/hour) was between 4 and 6 atmospheres of nitrogen pressure. By heat treating the powder in vacuum at 1800 C., in the same manner described by Long and Foster, they reduced the oxygen content to 0.82 percent and other impurities to 0.01 percent level. Sato, T. and Iwata, M., "Preparation of AlN by Electric Arc Method", Nippon Kagaku Kaishi, 1869-1873, (1973).
Other techniques used include thermal decomposition of AlCl.sub.3 NH.sub.3 complex and reduction of AlP by ammonia. These methods are likely to result in chlorine or phosphorous impurities in the AlN powder. S. Iwama describes a process for producing ultrafine powders of AlN by a reactive gas evaporation technique. Powdered aluminum nitride is produced by evaporating aluminum in an ammonia gas. An electron beam is used for heating. Use of nitrogen gas does not result in aluminum nitride. Journal of Crystal Growth 56 (1982) p. 265-269. Recently, Huseby described a method which involves the reaction of AlF.sub.3 and NH.sub.3 as shown in reaction (4) EQU AlF.sub.3(s) +NH.sub.3(g) .fwdarw.AlN.sub.(s) +3HF.sub.(g) ( 4)
The particles of AlN powder produced by this method were rod shaped ranging in size from 1 to 10 microns. The oxygen content varied from 0.17 to 0.30 percent. Huseby, I. C., "Synthesis and Characterization of a High-Purity AlN Powder", J. American Cer. Soc. 66 [3], 217-220 (1983).
AlN is an electrically insulating ceramic material with very high thermal conductivity (&gt;80 W/mK). To prepare such ceramics, however, it is required that the oxygen impurity must be reduced to the lowest possible level. A new process for reacting Al.sub.2 S.sub.3 with NH.sub.3 to produce high purity AlN is revealed below. The use of aluminum sulfide as the precursor material has not been previously reported to the knowledge of the inventor and represents a novel approach.